1. Field of the Invention
The invention relates generally to the field of gas and liquid chromatography and specifically to the design of chromatographic column assemblies which facilitate the heating of chromatographic columns using microwave heating methods.
2. Statement of the Problem
Gas and liquid chromatography are physical methods for the separation, identification, and quantification of chemical compounds. These methods are used extensively for applications that include the measurement of product purity in analytical chemistry, the determination of environmental contamination, the characterization of natural substances, and the development of pharmaceuticals.
The fundamental methods used in gas and liquid chromatographs to separate chemical constituents are similar. A sample mixture is injected into a flowing neutral carrier stream and the combination then flows through a tube or chromatographic column. The inner surface of the column is coated or packed with a material called the stationary phase. As the sample mixture and carrier stream flow through the column, the components within the mixture are retained by the stationary phase to a greater or lesser degree depending on the relative volatility (in the case of gas chromatography) or the relative solubility (in the case of liquid chromatography) of the individual components and on their respective affinities for the stationary phase. When the individual mixture components are released into the carrier stream by the stationary phase, they are swept towards the column outlet where they are detected and measured with a detector. Different chemical compounds are retained for different times by the stationary phase. By measuring the retention times, the specific compounds in the mixture can be identified. The relative concentration of the compounds is determined by comparing the peak amplitudes measured with the detector for each compound. The primary difference between gas and liquid chromatography is the mode of separation. In gas chromatography, the sample is volatilized and propelled down the analytical column by a moving stream of gas. In liquid chromatography, the sample is dissolved and propelled down the analytical column in a moving stream of liquid. Another difference between gas and liquid chromatography is that the columns used in liquid chromatography are generally filled or packed with the stationary phase, while those used in gas chromatography can also have the stationary phase coated or bonded to the interior wall, instead.
GC and LC measurements are facilitated by the application of heat to the chromatographic column to change its temperature. The use of a heated column oven in gas chromatographic systems greatly increases the number of compounds that can be analyzed and speeds up the time required for each analysis by increasing the volatility of higher molecular weight compounds. Heating an LC column affects the relative solubility of the mixture's components in the two phases and can enhance the separation as well as improve the repeatability of the elution times of the component chemicals.
Many methods have been described for heating chromatographic columns. The simplest and most commonly used method utilizes resistive heating elements to heat air which is in turn circulated through an insulated oven in which the column is placed. For example, U.S. Pat. No. 3,527,567 to Philyaw et al. describes a GC oven heated with resistive elements.
The resistive element heating method has several limitations. To achieve even heating of the column, a large volume of air is rapidly circulated around the chromatographic column. In addition to heating the column, the air heats the oven itself. Because the thermal mass of the oven is much larger than that of the column, the rate at which the column can be heated is commensurately reduced. A related problem is cooling time. After heating the oven to a high temperature during an analysis, it takes significantly longer to cool the oven plus the column to their initial temperature so that the next sample may be analyzed than it would to cool the column alone. Together, these limitations reduce the throughput of the chromatograph.
Attempts to localize the resistive heat element onto the column itself so as to reduce or eliminate peripheral heating of the `oven` are described in U.S. Pat. No. 3,169,389 to Green et al., U.S. Pat. No. 3,232,093 to Burow et al., and in U.S. Pat. No. 5,005,399 to Holtzclaw et al. Each of these patents describe methods for directly wrapping or cladding the chromatographic column with a resistive heating element. Methods are also described for positioning the resulting metal clad column adjacent to a cooling source to decrease cooling times. This method of heating can be difficult to implement in practice because of uneven heating of the column due to local hot or cold spots in the resistive heating element surrounding the column. Uneven heating of the column in turn compromises the quality of the analysis.
Yet another limitation of all resistively heated chromatographic devices is that if operated improperly, they can be driven to temperatures higher than the maximum tolerated by a given column resulting in damage to or destruction of the column.
An alternative method for heating chromatographic columns is microwave heating as described in U.S. Pat. No. 4,204,423 to Jordan. Potential advantages of microwave heating are efficiency and selectivity. Suitable objects placed in a microwave oven will be heated when the oven is operated, but the temperature of the oven itself will not change. Microwave heating occurs in materials which absorb the microwave energy and convert it into heat. Current chromatographic columns are generally made of materials that do not absorb microwave energy at an appreciable rate. For example, most GC capillary columns are made of polyimide and fused silica. Consequently, such columns will not heat at an appreciable rate when placed in a microwave oven. The apparatus taught by Jordan is not practicable with these columns.
Jordan teaches that any column material can be placed in a microwave oven except for conductive materials such as metals which will reflect the electromagnetic energy (by shorting out the electric field) in the microwave oven, thus rendering it inoperable. Indeed any such non-metal material can be placed in a microwave oven, but they will not necessarily be heated by the oven.
U.S. Pat. No. 3,023,835 to Brashear describes an apparatus for heating packed chromatographic columns by exposing them to radio frequency (RF) radiation. Brashear describes heating chromatographic columns via dielectric heating or via inductive heating (i.e., magnetic heating). In the case of dielectric heating, Brashear specifies that the column and the packing filler are constructed of electrically insulating materials. Most insulating materials, including those used to make chromatographic columns, do not absorb electromagnetic energy at a high enough rate to make dielectric heating as taught by Brashear practical. In the case of inductive heating, Brashear specifies that: (1) the column is constructed of a metal containing some magnetic components to enable inductive heating to occur; (2) the filler contains a metal powder to promote heat conduction from the column into the filler; and (3) the metal powder may also be magnetic to promote local inductive heating. In practice, inductive heating of the filler would not occur inside the metal column because it would be shielded from the electromagnetic field by the metal column in which it is sheathed. Moreover, metal-filled packing material inside columns is not generally a good scheme. The sample material passing down the column can be exposed to the metal. If the metal is not chemically inert, then some components of the sample can react with the metal thus distorting the resulting chromatogram.
Neither of the packed column constructions described by Brashear would be of practical usage in a microwave heating apparatus as taught by Jordan where the whole of the column is placed inside a cavity and exposed to high intensity electromagnetic radiation. The insulating low-loss column would not heat rapidly enough to be of practical use. The metal column would short out the electric field to such a significant extent that the microwave oven would not function properly and the column, if heated at all, would not be heated evenly.
3. Solution to the Problem
Microwave heating occurs in materials having dielectric or magnetic loss factors sufficiently high that when exposed to high frequency electromagnetic radiation these materials absorb sufficient energy from the electric or magnetic field to heat at an appreciable rate. Materials with high dielectric loss factors absorb energy from electric fields. Conversely, materials with high magnetic loss factors absorb energy from magnetic fields. Microwave heating apparatuses have both electric and magnetic fields within them so either loss mechanism will result in heating.
Food is heated well in microwave ovens because it contains water. Water has a high dielectric loss factor at microwave frequencies and is thus readily heated by energy absorbed from the electric field. The water in turn heats the rest of material in the food through conventional heat conduction and convection mechanisms. In order for a GC or an LC column to be heated in a microwave heating apparatus, it too must contain or be adjacent to a material which absorbs electromagnetic energy through dielectric or magnetic loss mechanisms. This material can then heat the column.
Chromatographic columns can be modified in one of two ways to make microwave heating practical. First, the column itself can be modified to incorporate a microwave absorbing material such that direct heating of the column is achieved. Second, the column can be positioned adjacent to (i.e., in contact with, or in close proximity to) a microwave absorbing material in an assembly that provides adequate heat transfer so that the column is heated by the microwave absorbing material via conduction or convection.
By carefully selecting the microwave absorbing material(s) used in chromatographic columns assemblies, intrinsic over-temperature protection can be built into the column so that it cannot be heated in a microwave heating apparatus to a temperature beyond a desired maximum temperature. Dielectric and magnetic loss factors are usually temperature dependent. If a microwave absorbing material is selected for which the loss factor decreases rapidly at the maximum desired column temperature, then the column assembly using this material cannot be heated appreciably above the desired temperature in a microwave heating apparatus. One way to achieve this is to select a ferromagnetic absorbing material having the correct Curie temperature. The loss factor of a ferromagnetic absorber decreases dramatically at its Curie temperature.
As compared to conventional resistively heated chromatographic systems, a microwave-absorbing column assembly heated in a microwave apparatus can be heated much more quickly and efficiently and subsequently cooled more quickly because: (1) the total thermal mass of the material to be heated or cooled is much smaller than in a conventional oven; and (2) the microwave energy selectively heats the desired material and not the whole of the oven.
Because the thermal mass of a microwave-absorbing column assembly is so small it could easily lose significant fractions of its heat to the environment while it is being heated. This undesired heat flow can limit the heating rate of the column and in some circumstances the cooling rate as well. To help control this heat flow, microwave-absorbing columns assemblies can be enclosed in insulating materials to better isolate them thermally from their environment. Standard chromatograph columns are not insulated in this manner from the oven environment. Materials used to insulate a microwave absorbing column assembly must be selected so as not to disturb the heating of the column by the microwave heating apparatus. To allow the heat to escape from the column when cooling is desired, the column insulator can be designed to allow air or some other heat sink material to be circulated around the column when needed to cool the column assembly.